An instrument of this kind is known from an article in the magazine "SCANNING", Vol. 8, pp. 285-293, by J. Zach and H. Rose, entitled "Efficient Detection of Secondary Electrons in Low-Voltage Electron Microscopy".
Instruments of the kind set forth are known as Scanning Electron Microscopes (SEM). In a SEM a region of a specimen to be examined is scanned by a primary beam of electrons of an energy of the order of magnitude of from 1 to 100 keV. Secondary Electrons of substantially lower energy are then released in the specimen, for example an energy of the order of magnitude of from 5 to 50 eV. The energy and/or the energy distribution of these secondary electrons provides information as regards the nature and composition of the specimen. Therefore, it is useful to provide a SEM with a detection device for secondary electrons. These electrons are released at the side of the specimen at which the primary beam is incident, after which they return, against the direction of incidence of the primary electrons, approximately along the field lines of the focusing lens. When a detector is arranged in the vicinity of the secondary electrons thus moving back, such a detector will collect the secondary electrons and output an electric signal which is proportional to the flow thus detected, which signal is then used to form the (secondary electron) image of the specimen in known manner.
The detector should satisfy inter alia the condition that it may not introduce significant imaging faults (geometrical as well as chromatic) in the primary beam so as not to increase the cross-section of this beam at the area of the specimen surface, which would degrade the resolution of the SEM. Furthermore, influencing of the direction of the primary beam by the presence of the detector is undesirable.
The cited article in the magazine "SCANNING" (notably page 286, section 2: "The Dipole Detector") describes a detector in which the secondary electrons are deflected and collected by a combination of a homogeneous electric field and a homogeneous magnetic field which extends perpendicularly thereto. Such a field combination has a few drawbacks. A first drawback resides in the fact that there are circumstances in which the presence of a magnetic field in a SEM is not desirable, for example when this field extends in the vicinity of the magnetic field of the focusing lens. A further drawback resides in the fact that such a system often introduces a chromatic error in the primary beam which cannot be ignored; additional correction must then be provided. This additional correction is realised in said article by means of an additional Wien filter having a polarity which opposes that of the detector; this filter is arranged above the focusing lens (see the last paragraph of section 2 of the cited article).